Marquand as a director is a lot like Gareth Edwards. Good with action and special effects, lackluster on characters. Return of the Jedi has that aspect to it that I don’t think would have been there without him.
I would say the opposite. Before ROTJ, Marquand mostly directed films based on character-driven stories/scripts. Also, people tend to forget that it is the story and script that essentially define the characters and how much focus is on them (which were basically done by Lucas and Kasdan in both films). I am pretty sure Marquand did his best to try to influence Lucas and Kasdan to put as much focus on characters as possible.
That being said, still, ROTJ has by far the most character development in the entire OT.